C-H bond acidity

Insertion Reactions. Isocyanates also may undergo iasertion reactions with C—H bonds. Acidic compounds, such as 1,3-dicarbonyl compounds (6), react readily at room temperature to form carboxyamides. At higher temperatures carboxyamides frequentiy undergo secondary reactions leading to cyclized products (33,34). 

The coordination of aromatic rings to transition metals, such as in h -C H A(CO)3, increases C—H bond acidity enough to allow C—H bond exchange with EtOD ... 

The polarized multiple bond makes adjacent C-H bonds acidic deprotonation generates the enolate, an allylic electron source. Often there is more than one possible regiochem or stereochem of the enolate, which can be selected by reaction conditions (see Section 9.3). 

Figure 7.7 The stereoelectronics of the allene system where the C-H bond is rigidly aligned with the vicinal Tt-system dramatically increases C-H bond acidity. ...

Their remarkable electron-withdrawing properties that cause strong C-H bond acidities in the a-positions and make them excellent leaving groups can be enhanced by the carbon substituent on sulfur, as the trifluoromethyl sulfonates ( triflates ) clearly indicate. 

Base is chosen so as to favor enolate formation. Acidity of C-H bond must be greater (lower pKa value) than that of the conjugate acid of the base (C S table 1.1, pg 3)... 

The C—H bonds of hydrocarbons show little tendency to ionize and alkanes alkenes and alkynes are all very weak acids The acid dissociation constant for methane for exam pie IS too small to be measured directly but is estimated to be about 10 ° (pK 60)... 

Using the relationship from the preceding section that the effective electronega tivity of carbon m a C—H bond increases with its s character (sp < sp < sp) the order of hydrocarbon acidity behaves much like the preceding methane ammonia water hydrogen fluonde series... 

Our experience to this point has been that C—H bonds are not very acidic Com pared with most hydrocarbons however aldehydes and ketones have relatively acidic protons on their a carbon atoms pA s for enolate formation from simple aldehydes and ketones are m the 16 to 20 range... 

Birds excrete nitrogen as uric acid Uric acid is a purine having the molecular formula C5H4N4O3 it has no C—H bonds Write a structural formula for uric acid... 

As a class of compounds, nitriles have broad commercial utility that includes their use as solvents, feedstocks, pharmaceuticals, catalysts, and pesticides. The versatile reactivity of organonitnles arises both from the reactivity of the C=N bond, and from the abiHty of the cyano substituent to activate adjacent bonds, especially C—H bonds. Nitriles can be used to prepare amines, amides, amidines, carboxyHc acids and esters, aldehydes, ketones, large-ring cycHc ketones, imines, heterocycles, orthoesters, and other compounds. Some of the more common transformations involve hydrolysis or alcoholysis to produce amides, acids and esters, and hydrogenation to produce amines, which are intermediates for the production of polyurethanes and polyamides. An extensive review on hydrogenation of nitriles has been recendy pubHshed (10). 

Oxaziridines substituted in the 2-position with primary or secondary alkyl groups undergo decomposition at room temperature. In the course of some weeks, slow decomposition of undiluted compounds occurs, the pattern of which is analogous to that of acidic or alkaline N—O cleavage (Sections 5.08.3.1.3 and 4), Radical attack on a C—H bond in (109) effects N—O cleavage, probably synchronously (57JA5739). In the example presented here, methyl isobutyl ketone and ammonia were isolated after two hour s heating at 150 °C. 

Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

The monofluoromethylene group and difluoromethyl group m 1H perfluoro-alkanes and -cycloalkanes are oxidized at the C-H bond to perfluoroalkyl and perfluorocycloalkyl fluorosulfates by anodic oxidation m fluorosulfonic acid [J, 4] Two modifications of the method are used ox idation by fluorosulfonyl peroxide generated pnor to the reaction [J] (equation 2A) and direct electrolysis m the acid [i, 4] (equabons 2B and 3)... 

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